In wireless communication systems, antenna arrays are used at devices on one or both ends of a communication link to suppress multipath fading and interference, and to increase system capacity by supporting multiple users and/or higher data rate transmissions.
For example, in frequency division duplex (FDD) systems, a base station (BS) device and each of a plurality of mobile station (MS) devices are equipped with a plurality of antennas. In general, the BS device may be referred to as a first wireless communication device and an MS device may be referred to as a second wireless communication device, or vice versa. The term “downlink” is used herein to refer to a transmission from a BS device to an MS device, and the term “uplink” is used to refer to a transmission from an MS device to a BS device.
One advantage of systems in which a BS device and the MS device both have a plurality of antennas is that transmit signals can be beamformed from one device to the other. When beamforming is employed, each of the transmitters in one device transmits the same signal but with different amplitude and phase through the respective antennas to the other device. Beamformed signals improve the signal-to-noise ratio (SNR) at the receiving device by exploiting the multipath effects of the communication channel between the two devices.
Another advantage of systems in which a BS device and an MS device both have a plurality of antennas is that they can be configured for multiple-input multiple-output (MIMO) communication. Specifically, multiple signal streams may be simultaneously transmitted between devices configured for MIMO communication, thereby substantially increasing the data rate capacity of the system on a frequency channel. Thus, while beamforming involves transmitting the same signal via multiple antennas from one device to another device, MIMO communication involves transmitting multiple signal (data) streams via multiple antennas from one device to another device.
Combining beamforming and MIMO techniques (referred to herein as beamforming+MIMO) is seen as an attractive way to improve the capacity of a communication channel. An important aspect in the implementation of a system that combines beamforming and MIMO techniques is the computation of the weights that are applied to the signal streams to be transmitted from one device to another device. Knowledge of the channel conditions is needed to compute weights that will achieve a desirable reception quality of the signal streams at the destination device.
Generally, there are two types of schemes to obtain the channel information (conditions). One technique is referred to as a “closed-loop” technique whereby the destination device detects the channel information (e.g., downlink channel information where the destination device is an MS device) based on specifically designed downlink signals transmitted by the BS device and transmits a signal (as feedback) containing the channel information to the BS device. This requires processing on the MS device and additional overhead on the uplink channel. Another technique is referred to as channel sounding and which uses uplink signals received from an MS device at a BS device to estimate the spatial signature of the plurality of antennas of the BS device to the plurality of antennas of the MS device. Channel sounding is not an attractive option in FDD systems because the uplink and downlink signals are separated in frequency such that the downlink and uplink channels are uncorrelated.
A need exists to derive knowledge of a communication channel between a first device having a plurality of antennas and a second device having a plurality of antennas when the frequencies of the uplink and downlink are separated. Using this knowledge of the channel, more accurate values for beamforming weights can be computed for transmitting one or more signals from a plurality of antennas of the first device to a plurality of antennas of the second device, which is particularly desirable in an FDD communication system configured to use beamforming+MIMO techniques.